1. Field of the Invention
The present invention relates to Intelligent Network (IN) based telecommunications networks.
2. Description of Related Art
There is currently considerable activity in technical standards bodies (CCITT, ISO etc) and elsewhere developing Intelligent Network (IN) standards and implementation. Simply, the basis of IN is to separate service provision from basic switching functionality in telecommunications networks. Typically, the processing required to implement network services is provided by computers which are independent from the network switching infrastructure. This allows for rapid creation and introduction of new services without being excessively constrained by network switches.
In present day IN standards (CCITT Capability Set 1-CS1) and implementations the platform for launching all IN services is a "Basic Call State Model" (BCSM) implicitly embedded in the software of each IN switch in a network. The BCSM defines a finite number of states in the lifetime of a telecommunication call e.g off-hook, collect digits, etc. Within the BCSM a number of Detection Points (DPs) are identified, at which certain events can trigger the switch to halt its call processing and refer to service control entities for further instructions. Service control entities are able to modify the call processing by, for example, providing the switch with routing information derived from a variety of parameters, e.g dialled number, time of day, day of week, subscriber preferences, etc, etc, thereby offering the possibility of developing a wide variety of user services.
IN has great potential for offering advanced network services in the future. It has been proposed to use IN principles and implementation as the basis for future mobile telecommunication systems, and while a number of IN enhancements needed to achieve this have already been identified, further changes will be required.
In the mobile environment the IN approach may be applied to the implementation of mobility services such as location management and handover, as well as conventional user services. Thus the processing required to achieve these mobility services is moved out of the network switches and into service control entities. Doing this reduces the impact of mobility on switch design, i.e there may be no need for dedicated mobile switching centres, and offers opportunity for providing mobility on a number of network platforms. By contrast, the existing GSM cellular radio system embeds its mobility functions in the switching infrastructure, e.g in mobile switching centres.
Present day IN services (CS1) are all built on the platform of the Basic Call State Model, (BCSM) in which the current state of a call in progress between two user terminals is stored, and this state is used to control other services. However, there is a problem in implementing some basic mobility services on the Basic State Model platform. Mobility services are inherently independent of calls in progress. In particular handover monitoring and location undate can occur at any time, independently of any prompt from either the user or the network management function. Moreover, some services require interactions involving only one user terminal, which interacts with an internal network termination such as a Location Register, or a Voice Message Centre (VMC). In this specification the term "network termination" is used to embrace both user terminals (e.g. telephone handsets) and such internal network terminations.
Handover is a function which typically takes place during a call in response to changes in signal quality. If the system detects a deterioration in the link between the mobile unit and the base station with which it is currently in communication, the system attempts to locate another base station which can provide a better link, establish a link with this second base station and to break the link with the first base station. This operation should take place without any action necessary on the part of the user (who should ideally not even be aware that it is happening). Moreover, user services such as voice messaging, which are controlled centrally by the service control entities in the network management centre, should be able to function unaffected whilst this process is taking place.
In the location update procedure a base station or a geographical group of base stations transmit a location identification transmission. Mobile units in standby mode (i.e. able to receive calls but not currently engaged in one) periodically monitor this transmission to determine their approximate location. Should the location identification change, the mobile unit reports its new location to the network. When a call attempt to the mobile unit is made, the system initially pages the mobile unit only in the area in which the mobile unit was last reported (i.e. it is paged only from the base station(s) having that location identification).
Clearly, changes in location state or handover may take place at times other than when a call state is changed (set up or cleared) and so the mobile unit's location state changes and handovers cannot be handled by the known Basic Call State Model.